1. Field of the Invention
The present invention is directed to a method and apparatus to reduce the concentration of dissolved species in water and, more specifically, to a method and apparatus for microfiltration.
2. Description of the Related Art
Water may contain a variety of materials including, among other contaminants, divalent cations that increase the "hardness" of the water. The most prevalent divalent cations contributing to hardness are calcium and magnesium. Hard water may present a variety of problems for users in residential or industrial use. In addition, due to their negative solubility coefficients, calcium and magnesium in the water may precipitate out and form scale on heat exchange surfaces. Elevated levels of hardness may make it more difficult to remove other undesirable components such as other ionic compounds or organic compounds and may foul water treatment systems. For example, reverse osmosis membranes may be fouled by even relatively low levels of hardness.
Several techniques exist for the reduction of water hardness. For instance, calcium and magnesium cations may be removed by ion exchange where calcium and magnesium ions are exchanged for alternative cations that have a less deleterious effect in the water. Such methods require a source of alternative cations and are limited by the relative affinity of various ion exchange substrates to the competing cations as well as by the kinetics of the ion exchange itself. After use, these ion exchange substrates, for example, ion exchange resins, must be either recharged or replaced.
Calcium, magnesium and silica may also be removed by precipitation or "lime softening." Generally, either lime or soda ash, or both, is added to the water to reduce the amounts of calcium, magnesium and silica that are in solution to about the level of solubility of calcium carbonate and magnesium hydroxide, respectively. The precipitated matter is then removed from the water either by filtration or by settling.
Traditionally, lime softening involves the conversion of soluble salts in water into non-soluble salts. In some non-critical applications, there is no need to separate the sediments from the water. In more critical applications, separation and removal of the non-soluble salts is mandatory. Therefore, separation processes, as well as chemical processes, may be involved in lime water softening.
A conventional sedimentation following flocculation and agglomeration is currently employed as the most common method of separation.
A significant improvement of the separation technique was achieved by using membranes as a separation media. In 1983, D. Comstock, et al. published results of the "Hydroperm" process using cross flow microfiltration as a separation device after the lime softening process. Kedem, et al., in U.S. Pat. No. 5,152,904, describe water treated by the addition of sodium hydroxide being contacted with large excess of specific-sized calcium carbonate crystals followed by separation.
The conventional methods of water softening, however, suffer from high operating and capital cost, limited throughput, or insufficient reduction in concentrations of various ions such as calcium, magnesium or silica.